Sajjan Kumar was a symbol of 1984 anti-Sikh riots.
Indira Gandhi, 66, was murdered by the two guards as she walked in the garden of the prime minister's residence on the morning of 31 December.
The assassination followed Operation Blue Star earlier in the same year, which saw Indian military try to flush out Sikh separatists from Amritsar's Golden Temple, the faith's holiest shrine.
Reacting to the verdict, the Shiromani Akali Dal attacked the Congress.
More than 3,000 Sikhs were killed just in New Delhi in the riots.
Sajjan Kumar, 73, the senior Congress leader sentenced to life for his role in the 1984 anti-Sikh riots, has been a key player in Delhi politics since the 1970s.
The court also directed them to surrender by December 31 and not leave the city before that.
The CBI had filed an appeal challenging the acquittal of Sajjan Kumar and said that the trial court "erred in acquitting Sajjan Kumar as it was he who had instigated the mob during the riots".
Jagdish Kaur, whose son and husband were among five family members brutally killed, described the verdict as "a little balm applied after a long time to our scars".
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"At least one high-profile accused will now go to jail", she said. "Sajjan Kumar's conviction by the Delhi High Court is a delayed vindication of Justice". It said such cases "are indeed extraordinary and require a different approach to be adopted by the courts".
Congress MP Abhishek Manu Singhvi said Kumar's sentencing should not be linked to the "political atmosphere prevailing in the country".
The Congress on Monday said the conviction of its leader Sajjan Kumar in a 1984 anti-Sikh riots case should not be politicised and that the law should take its own course, PTI reported.
The High Court while reading the judgement said, "In the summer of 1947, during partition, several people were massacred". "A majority of the perpetrators of these horrific mass crimes enjoyed political patronage and were aided by an indifferent law enforcement agency", the judges said.
Congress president Rahul Gandhi.
The convicts had challenged their conviction and sentencing by the trial court in May 2013. Other people who were accused in the riots were given positions in Gujarat.
Finance Minister Arun Jaitley was apparently referring to Kamal Nath, who took oath as the chief minister of Madhya Pradesh, in a ceremony attended by top opposition leaders. Both deny any involvement. There are significant Sikh communities in Canada, the UK, US and Australia.
The Delhi High Court in its order today also convicted Captain Bhagmal, Girdhari Lal and Balwan Khokar, they have also been given life sentences while Kishan Khokar and Mahender Yadav have been sentenced to 10 years in prison.
Thousands of Sikhs, whose family members were killed by rampaging mobs in November 1984, have welcomed this judgement.
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